I would forgo the grow bags, that is if you would like some flavor in your berries! in my experience I got a lot of berries that were no better than store bought, 1gl pot with one plant per pot work nice. I don't know about z6 but the plants I had in the 1gl pots took temps down to 10 degrees without any problem, I just don't know how low they can go. I have not tried Alpine yet, tried growing some seed with no luck at this time.

I left two hanging baskets out that were full of strawberries the year before and every single plant died. They all froze! If you're doing that, I suggest storing them in the garage or some place a bit warmer than outside during the winter.

I've found that the best berries do come from berries planted in the ground. I didn't quite as large a harvest from the plants in a pot, but the few berries were pretty good. They didn't produce at the same time, so I couldn't taste test.